This invention relates to processes for radical-mediated polymerization of unsaturated species, and for the control or limitation of the molecular weights of the polymeric products produced from such processes. Polymers of limited molecular weights, or oligomers, are useful as precursors in the manufacture of other polymeric materials, and as additives in plastics, elastomerics, and surface-coating compositions, as well as being useful in their own right in many applications.
In conventional polymerization practice, manufacture of oligomers requires the use of a free radical source, the initiator, and of a chain-transfer agent. The chain-transfer agent controls the molecular weight of the polymer molecules by reacting with the propagating radical species to terminate its growth and later initiates a new polymer chain thus transfering the growth process from one discrete polymer molecule to another discrete polymer molecule. At least a part of the chain-transfer agent is incorporated into the polymer molecules, and thus is consumed during the process which may result in undesirable increases in average molecular size and polydispersity of the product. The incorporated residue of the chain transfer agent can also lead to undesirable end-groups on the polymer.
Transition metal complexes are widely used as polymerization agents, but principally as catalysts or coagents in redox radical initiation systems, for example, with organic peroxides for the curing of styrene/unsaturated polyester formulations, or in the initiation of ionic polymerization, for example, of epoxides. Transition metal complexes have also been used for the control of stereochemistry and the inhibition of crosslinking (gel formation) in the polymerization of butadene and other olefins. Transition metal complexes, including species related to those described in the specification have also been used as catalysts for the hydrogenation or oxidation of organic compounds.